The present invention relates to a recording apparatus for producing hard copies for video systems, computers or the like.
Recently, there has developed a strong desire to produce a hard copy having high quality of a coloured picture in a shorter time, and in order to satisfy this desire, apparatus primarily of a thermal printing type are now being put in practical use. For recording a coloured picture by means of thermal printing, there is adopted a three colour sequential printing system in general, in which an ink sheet has three sections of three colours corresponding to three color images of the picture, and three printing operation each corresponding to each of three colours are carried out on a single picture section of a recording paper sheet. In this system, it is required to return the recording paper sheet three times to the same position relative to a thermal head of a recording device. In the conventional recording apparatus of the above-mentioned type, the recording paper sheet conveying systems are roughly grouped into two types.
A recording apparatus according to a first type conveying system of a prior art is shown in FIG. 1. A drum 104 having an elastic surface 104a is rotated in a direction indicated with an arrow 1a, while the leading edge 101a of a recording paper sheet 101 is gripped by a gripping device 104b and the recording paper sheet is wound around the drum 104. During the first revolution of the drum 104, an image of the first colour is recorded in a manner such that heat generating elements 103a of a thermal head 103, which makes pressure-contact with the drum 104 with an ink sheet 102 and a recording paper sheet 101 interposed therebetween, are selectively energized for thermal printing. In a similar way, images of the second colour and third colour are sequentially recorded on the same section of the recording paper sheet during the succeeding two revolutions of the drum 104 for completing a full-colour recording of one picture.
As mentioned above, in the first prior art type conveying system, the return of the recording paper sheet to its original position is made by rotating the drum 104 successively by three revolutions in one direction. As a result, there is obtained an advantage that no reciprocal return time of the recording paper sheet may be eliminated, resulting in a shorter recording time.
Next, a recording apparatus according to a second prior art type conveying system is shown in FIG. 2. A recording paper sheet 201 is reciprocally conveyed by means of a driving capstan roller 205 rotatable in both rotating directions and a pinch roller 206 in pressure contact with the capstan roller with the recording paper sheet holded therebetween. As conveying the recording paper sheet 201 in a direction indicated with an arrow 2a and pressing a thermal head 203 onto a platen roller 204 having an elastic surface 204a with an ink sheet 202 and the recording paper sheet 201 interposed therebetween, heat generating elements 203a are selectively energized or controlled in its heat amount, thereby producing a picture having a first colour of the ink sheet 202. Next, the thermal head 203 is separated from the platen roller 204 and the capstan roller 205 is rotated in the reverse direction, returning the recording paper sheet 201 in a direction indicated with an arrow 2b to the same position as before recording. Then, the thermal head 203 is again pressed against the platen roller 204, and as conveying the recording paper sheet 201 in a direction indicated with an arrow 2a, an image of a second colour is recorded on the recording paper sheet which has been already recorded with the image of the first colour. In a similar way, an image of a third colour is recorded on the recording paper sheet already recorded with the first and second colours, thereby completing a recording of a full-coloured picture.
As mentioned above, since, in the second prior art type conveying system, the recording paper sheet 201 is conveying by means of the rigid capstan roller 205, the convey speed of the recording paper sheet 201 is relatively stable. As a result, a fine picture having decreased slippage among three colours can be obtained.
However, the recording apparatus of the prior art include the following problems.
In the above-mentioned first prior art type conveying system, the peripheral speed v' of the drum 104 measured at the pressure contact portion of the thermal head 103 is greater than the speed v measured at the non-contact portion of the drum including the portion occupied by the gripping device 104. Accordingly, the conveying speed of the recording paper sheet is dependent upon the peripheral speed v' at the pressure contact portion. However, the peripheral speed v' at the pressure contact portion is unstable, and varies considerably depending upon the elasticity of the elastic body 104a or the contact pressure of the thermal head 103. In consequence, the conveying speed of the recording paper sheet can not be maintained at a precisely constant value, thereby causing an irregular density of the recording or positional slippage among the three colour images in recording, resulting in deterioration of picture quality.
Further, in the above-mentioned second prior art type recording system, the recording paper sheet 201 is reciprocally conveyed. Therefore, there are required a rather longer return time of the recording paper sheet and repetitions of contact and separation of thermal head 203, causing a problem of a longer recording time.